To provide a device for adjusting tension of a timing belt capable of preventing resonance of the timing belt with an engine, improving the lifetime of the timing belt and reducing the noise by suppressing an increase in tension of the timing belt, or reducing the number of idler pulleys or eliminating the use of the idler pulleys.

The timing belt tension adjusting device includes a pulley 10 around which the timing belt is extended, a housing 20 (a rotary member constituting a tension adjustment means) for adjusting the belt tension by displacing the central position of rotation of the pulley 10, a vane member 30(a vehicle side member constituting the tension adjustment means), and an ECU 53 for varying the frequency of resonance of the timing belt by controlling the rotary action of the housing 20 when the timing belt resonates with the engine. The ECU 53 controls the rotary action of the housing 20 based on the engine speed so that the resonance frequency may not correspond to the frequency of vibration of the engine.